Custom validators in Flutter - UI Render Trace

Component - Custom validators

This UI component shows a simple form with a text input field that uses a custom validator function. The validator checks if the input text meets specific rules before allowing submission.

Widget Tree

Scaffold
├─ AppBar
│  └─ Text
└─ Padding
   └─ Form
      └─ Column
         ├─ TextFormField
         └─ ElevatedButton

The Scaffold provides the basic page structure with an AppBar at the top showing a title. Inside the body, Padding adds space around the form. The Form widget holds a Column that stacks the TextFormField input and a submit ElevatedButton vertically.

Render Trace - 6 Steps

Step 1: Scaffold

Step 2: Padding

Step 3: Form

Step 4: Column

Step 5: TextFormField

Step 6: ElevatedButton

State Change - Re-render

Trigger:User taps the 'Submit' button

Before

Form input is empty or invalid

→

After

Validator runs, shows error message if input invalid, or allows submission if valid

Re-renders:TextFormField rebuilds to show error message if validation fails

UI Quiz - 3 Questions

What happens when the user taps the Submit button with invalid input?

A The app closes immediately

B The TextFormField shows an error message below the input

C Nothing happens, the button is disabled

D The input text is cleared automatically

Key Insight

Using custom validators inside a Form widget helps ensure user input meets your app's rules before proceeding. The validator function runs when the user tries to submit, and the UI updates to show helpful error messages if needed. This improves user experience by giving immediate feedback.

Practice

1. What does a custom validator function in Flutter return when the input is valid?

easy

A. false

B. null

C. true

D. An empty string

Solution

1. Step 1: Understand validator return type

   A custom validator returns a String error message if input is invalid, otherwise null.

2. Step 2: Identify valid input return

   When input is valid, the validator returns null to indicate no error.

3. Final Answer:

   null -> Option B

4. Quick Check:

   Valid input returns null [OK]

Hint: Valid input returns null, errors return strings [OK]

Common Mistakes:

• Returning empty string instead of null
• Returning boolean true/false
• Returning error message on valid input

2. Which of the following is the correct syntax for a simple custom validator in Flutter that checks if input is empty?

easy

A. validator: (value) => value.isEmpty ? null : 'Enter text'

B. validator: (value) { if (value.isEmpty) return 'Enter text'; }

C. validator: (value) => if (value.isEmpty) 'Enter text' else null

D. validator: (value) => value!.isEmpty ? 'Enter text' : null

Solution

1. Step 1: Check correct Dart syntax for validator

   The validator must return a String or null. validator: (value) => value!.isEmpty ? 'Enter text' : null uses a concise lambda with a ternary operator correctly.

2. Step 2: Identify null safety and return values

   validator: (value) => value!.isEmpty ? 'Enter text' : null uses value! to assert non-null and returns error string if empty, else null.

3. Final Answer:

   validator: (value) => value!.isEmpty ? 'Enter text' : null -> Option D

4. Quick Check:

   Correct syntax uses ternary and returns null on valid input [OK]

Hint: Use ternary operator and return null for valid input [OK]

Common Mistakes:

• Missing return statement in block syntax
• Using if without else in lambda
• Returning null on empty input instead of error

3. Given this validator code inside a TextFormField:

validator: (value) {
  if (value == null || value.length < 5) {
    return 'Too short';
  }
  return null;
}

What happens if the user enters "abc"?

medium

A. The app crashes due to null error

B. The form accepts input with no error

C. The form shows error message 'Too short'

D. The validator returns null but error shows anyway

Solution

1. Step 1: Analyze input length check

   The validator checks if value is null or length less than 5, then returns 'Too short'.

2. Step 2: Apply input "abc" to condition

   "abc" length is 3, which is less than 5, so validator returns 'Too short'.

3. Final Answer:

   The form shows error message 'Too short' -> Option C

4. Quick Check:

   Input length < 5 triggers error message [OK]

Hint: Check input length against condition to predict error [OK]

Common Mistakes:

• Ignoring null check and assuming no error
• Confusing return null with error message
• Assuming validator crashes on short input

4. Identify the error in this custom validator code:

validator: (value) {
  if (value.isEmpty) {
    return 'Cannot be empty';
  }
}

medium

A. Using isEmpty on nullable value without null check

B. Missing return null for valid input

C. Returning a string instead of boolean

D. Validator should not use if statements

Solution

1. Step 1: Check null safety on value

   value is nullable String?, so calling value.isEmpty without null check causes error.

2. Step 2: Identify missing null check

   Must check if value is null before calling isEmpty to avoid runtime error.

3. Final Answer:

   Using isEmpty on nullable value without null check -> Option A

4. Quick Check:

   Always check for null before isEmpty on nullable String [OK]

Hint: Check for null before calling isEmpty on value [OK]

Common Mistakes:

• Assuming value is non-null without checking
• Not returning null for valid input (though important)
• Confusing return types of validator

5. You want to create a custom validator that checks if input is a valid email and also ends with ".com". Which validator implementation correctly combines these checks?

hard

A. validator: (value) {
if (value == null || !value.contains('@')) return 'Invalid email';
if (!value.endsWith('.com')) return 'Must end with .com';
return null;
}

B. validator: (value) => value!.contains('@') && value.endsWith('.com') ? null : 'Invalid email or domain';

C. validator: (value) {
if (value == null) return null;
if (!value.contains('@') || !value.endsWith('.com')) return 'Invalid email';
return null;
}

D. validator: (value) {
if (value == null || value.contains('@') && value.endsWith('.com')) return null;
return 'Invalid email';
}

Solution

1. Step 1: Check null and '@' presence

   validator: (value) { if (value == null || !value.contains('@')) return 'Invalid email'; if (!value.endsWith('.com')) return 'Must end with .com'; return null; } first checks if value is null or missing '@', returning 'Invalid email'.

2. Step 2: Check if ends with '.com'

   Then it checks if value ends with '.com', returning specific error if not.

3. Step 3: Return null if all checks pass

   If both checks pass, it returns null indicating valid input.

4. Final Answer:

   validator: (value) { if (value == null || !value.contains('@')) return 'Invalid email'; if (!value.endsWith('.com')) return 'Must end with .com'; return null; } -> Option A

5. Quick Check:

   Separate checks with clear error messages [OK]

Hint: Check conditions stepwise, return null only if all pass [OK]

Common Mistakes:

• Combining conditions incorrectly with && or ||
• Returning null on null input without error
• Not checking null before calling methods